In recent years, the voltage applied to a spark plug has been increased because of the increasing power of an internal combustion engine. Therefore, the level of radio noise (ignition noise) generated upon occurrence of spark discharge tends to increase. To reduce such radio noise, various techniques have been proposed (see, for example, Japanese Patent Application Laid-Open (kokai) No. H05-152053; Japanese Patent Application Laid-Open (kokai) No. H11-233232; and Japanese Patent Application Laid-Open (kokai) No. 2006-66086).
Generally, the level of radio noise generated by a spark plug can be reduced by increasing the resistance of a resistor disposed in a connecting portion that electrically connects a center electrode of the spark plug with its metal terminal. However, when the resistance of the resistor is increased in order to reduce the level of radio noise, ignition energy decreases, and this may cause deterioration of the sparking performance of the spark plug.
In view of the above problem, an object to be achieved by the present invention is to reduce the level of radio noise generated from a spark plug while suppressing deterioration of its sparking performance.